Several methods of manufacturing an SOI substrate by bonding have been disclosed. Three representative methods will be described below.
In the first method, two substrates are bonded while inserting an oxide film between them. Polishing and grinding are performed from one side to leave a substrate having a desired thickness on the oxide film (see M. Shinbo, K. Furukawa, K. Fukuda and K. Tanazawa, J. Appl. Phys., vol. 60, p. 2987, 1986). Based on this technique, several methods of thinning a substrate portion with high controllability have been proposed.
The second method uses porous silicon (see Japanese Patent Laid-Open No. 5-21338). In this method (ELTRAN (registered trademark)), an epitaxial silicon layer grown on a porous silicon substrate is bonded to a support substrate while inserting an oxide film between them. After annealing is executed to increase the bonding strength, the structure is cleaved and split by an external force along stress in the porous silicon layer. The porous silicon layer remaining on the surface of the layer transferred to the support substrate side is selectively etched, thereby obtaining an SOI substrate. In this method, a similar SOI substrate can also be obtained by grinding the bonded substrate stack from the back surface on the porous layer formation side to expose the porous silicon layer and then selectively etching the porous layer.
The third method uses hydrogen ion implantation (see Japanese Patent Laid-Open No. 5-211128). In this method (Smart Cut (registered trademark)), an oxide film is formed on at least one of two silicon substrates. In addition, hydrogen ions or rare gas ions are implanted from the front surface of one silicon substrate to form a micro-bubble layer (enclosed layer) in the substrate. After that, the ion-implanted surface is bonded to the other silicon substrate (support substrate) while placing the oxide film between them. Annealing is executed to peel one substrate thin from the micro-bubble layer serving as a cleavage plane. Annealing (bonding annealing) is further executed to increase the bonding strength, thereby obtaining an SOI substrate.
The common problem of these bonded SOI substrate stack manufacturing methods is how to control the bonding surface between the substrates. In order to control the bonding surface, it is important to increase the bonding strength. As a technique of increasing the bonding strength, a hydrophilic process performed on a silicon surface side when the silicon surface is bonded to a silicon oxide surface has been proposed (see Proc. 4th International Symposium on Si on Insulator Technology and Devices, May 6-11, 1990, Moutread). A technique of further increasing the bonding strength by activating the bonding surface has also been proposed (see G.G. Goetz, Electrochemical Society, Bonding Symposium 1991, Extended Abstract p. 65, and Japanese Patent No. 3294934). Note that the surface also becomes hydrophilic in a method disclosed in G.G. Goetz, Electrochemical Society, Bonding Symposium 1991, Extended Abstract p. 65, and Japanese Patent No. 3294934.
When water (physical adsorbed water, chemical bonding water, and the like) is excessively present on the bonding surface, the excessive water is trapped at the periphery of the substrate in a bonding process. This causes small voids (spaces) at the periphery, thus posing a problem. As a means for solving this problem, a method of making the silicon surface hydrophobic when the silicon surface is bonded to the silicon oxide film surface has been proposed (see Japanese Patent Laid-Open No. 9-331049).
If the silicon surface is hydrophilic when the silicon surface is bonded to the silicon oxide film surface, a high bonding strength can be obtained. Hence, a sufficient bonding strength can be advantageously obtained by annealing at a relatively low temperature in the annealing process after bonding. However, in this case, the voids caused by the excessive amount of the adsorbed water on the surface tend to be formed at the periphery of the bonding surface.
When the silicon surface is hydrophobic, no void is formed at the periphery of the substrate, advantageously. However, since the high bonding strength cannot be obtained, the annealing temperature must be set to be relatively high after bonding.